Subject: [mg124687] large lists with variable and if condition (vrs. 8.0.4)

From: Chris <kristophs.post at web.de>

Date: Tue, 31 Jan 2012 05:39:17 -0500 (EST)

Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Hi
I would like to construct a large list or large table with some data
and one unknown variable which I need for optimization purposes in say
a for loop. Constructing the whole list at each iteration is time
consuming and slows the optimization down.
First, I present some code that works just fine in order to get the
idea:
(*inputs)
leg = 100;
data = RandomReal[{}, leg];
(*constructing the list with the data in which g is a non-specfied
variable*)
Clear[list, list2, tab];
list = Table[(data[[i]] - data[[leg - i + 1]])/g, {i, 1, leg}];
(*specifying the function and the result*)
tab[k_] := list /. g -> k;
tab[1]
Now the following code does not work and I don't know why.
list2=
Table[If[Abs[(data[[j]]-data[[i]])/x]<1.,.75 (1.-((data[[j]]-
data[[i]])/x)^2),0.],{i,1,leg},{j,1,leg}];
mat[bw_Real]:=list/.x->bw
It seems to me, when looking at list2, that Mathematica is not
evaluating the when-part of the if-function when the if-part depends
on the unknown variable. Hence, an error message.
Constructing the list as:
mat[bw_Real]:=Table[If[Abs[(data[[j]]-data[[i]])/bw]<1.,.75 (1.-
((data[[j]]-data[[i]])/bw)^2),0.],{i,1,leg},{j,1,leg}];
is not an option because it slows things down.
Thanks for answer.